A solid polymer fuel cell comprises a membrane electrode assembly (MEA) comprising an electrolyte membrane (comprising an ion-exchange membrane) and catalyst layers and gas diffusion layers that are disposed on both sides of the electrolyte membrane, a separator laminated on the MEA, and the like. Catalyst layers may be formed on an electrolyte membrane first or on gas diffusion layers first. In the former case, on catalyst layers that have been formed on an electrolyte membrane that serves as a substrate, gas diffusion layers are laminated by heating and pressing, so that an MEA is obtained. In the latter case, gas diffusion layers are laminated in a way such that catalyst layers that have been formed on gas diffusion layers face both sides of an electrolyte membrane so that an MEA is obtained.
A catalyst layer contains electrode powder (catalyst supporting conductor) such as platinum-supporting carbon. As described above, a catalyst layer is formed by a method whereby electrode powder is disposed on an electrolyte membrane or a gas diffusion layer that serves as a substrate. An example of such method that has been conventionally carried out is so-called wet application, wherein an electrode ink is applied to a substrate by screen printing, a blade technique, roller coating, spraying, or the like. A method that has recently started to be adopted is a dry method for dispersing electrode powder toward an electrolyte membrane or a gas diffusion layer that serves as a substrate utilizing the electrostatic force or flow of gas (carrier gas) for the direct adhesion of the powder.
JP Patent Publication (Kokai) No. 2003-163011 A discloses a method of continuous production of MEAs by a dry method utilizing the electrostatic force. In such method, electrode powder is applied to a drum in a predetermined pattern to be electrically charged, and the electrode powder is transferred to a continuously supplied electrolyte membrane using the electrostatic force for fixation by heating and pressing. JP Patent Publication (Kokai) No. 2002-367616 A discloses a technique wherein platinum-supporting carbon that serves as electrode powder is electrically charged, the carbon is allowed to cumulate on a roller while being subjected to patterning via a control blade that controls the transferred pattern for transfer and fixation of the carbon on an electrolyte membrane, so that an MEA is obtained.
The present inventors have much experience with the production of MEAs by a dry method as described above. In such production process, when the transferred pattern becomes a complicated shape, the present inventors experienced deterioration in the product manufacturing accuracy. This was because variation of thickness or collapse of the outline of the pattern was found on a catalyst layer formed with electrode powder that had been transferred to an electrolyte membrane or a gas diffusion layer that serves as a substrate. Such problems can be resolved to some extent by increasing the applied voltage. However, the electric field is not allowed to be 3 kV/mm or more, at which level dielectric breakdown occurs. In addition, the flowing of a large current into electrode powder may cause ignition.
In a method described in JP Patent Publication (Kokai) No. 2002-367616 A, multiple ring electrodes are disposed to surround a hole formed on a control blade and electrode powder dispersion is carried out exclusively via a hole where positive potential is applied and not via a hole where negative potential is applied, so that it is possible to specify to which part of an electrolyte membrane electrode powder adheres. Therefore, it is expected that a catalyst layer having substantially uniform thickness would be formed. However, it is possible to form a control blade having such complicated structure, it is not practical because such blade increases the cost of an apparatus and requires complicated maintenance.
The present invention has been made under the circumstances described above. It is an object of the present invention to provide a method and an apparatus for forming a noble catalyst layer, wherein variation of thickness and collapse of the outline are extremely reduced on such catalyst layer that is transferred to a substrate via the electrostatic force using a conventionally used mesh-like screen, so that an MEA having a high product manufacturing accuracy can be obtained.